Pleura (πλευρά) means a side, rib, or flank of the body. In the NT it is used exclusively to refer to the side of Jesus Christ — both when pierced on the cross and when shown to Thomas after the resurrection. The word is the origin of the English medical term 'pleural.'
In John 19:34, a soldier pierces Jesus' side with a spear, and blood and water flow out. The blood and water symbolize atonement (blood) and cleansing/new life (water). In John 20:20 and 20:25-27, Jesus shows His side to the disciples and to Thomas, offering His wounds as proof of resurrection. The risen Christ still bears the marks of crucifixion. Church fathers connected the pierced side of Christ to Genesis 2:21-22, where God formed Eve from Adam's side — the Church, the Bride of Christ, was 'born' from the wounded side of the second Adam.